The present invention relates to nuclear radiation measuring devices of the type used to measure the amount of radiation exposure delivered over a period of time. More specifically, it relates to an improved direct-reading dosimeter for providing both a combined and a separate measurement of low-level gamma and neutron exposure.
As nuclear technology has become more widespread in our society, so has the desire for radiation measuring devices that can detect even the slightest levels of exposure that an individual has received over any given time. Moreover, those handling radioactive materials for testing and developing nuclear technology and those exposed to nuclear weaponry, nuclear power plants, and workers in the field of nuclear medicine, have long recognized the need for a dosimeter that can measure radiation doses at levels as low as the known average daily background level.
Currently, there are few devices that can conveniently and inexpensively provide such low-level measurements. One device, a Photo-Multiplier/Scintillator System utilizes the immediate conversion of ionizing radiation energy into visible light energy to measure radiation exposure. Another device, a thermoluminescent dosimeter, utilizes thermoluminescent materials (e.g. LiF) to detect exposure levels. In such a device, radiation raises the electrons in the thermoluminescent material to an excited energy level. These electrons remain excited until the material is exposed to high temperature. Consequently, radiation exposure levels can be determined by heating the exposed crystals and measuring the amount of light emitted as the excited electrons drop back to their ground state. To those who desire quick measurements, in real time, while they operate in an environment suspect of radiation, both the Scintillator System and the thermoluminescent dosimeter are inadequate (inconvenient and time consuming).
A more preferred method of measuring exposure in real time is a direct-reading dosimeter or exposure meter. One such device, a 200 mR (5.16 * 10.sup.-2 C/Kg) Carbon Fiber Dosimeter, is disclosed in U.S. Pat. No. 4,306,154, issued Dec. 15, 1981 by Williams et al and incorporated herein by reference. This device utilizes an ionization chamber, an electrically charged fiber electrometer, and a viewing means to measure exposure. Radiation passing through the ionization chamber discharges the electrometer, thereby moving the charged electrometer fiber in proportion to the amount of radiation passing through the chamber. Thus, an individual wearing the dosimeter can determine his own exposure by simply looking through the viewing means at the electrometer fiber deflection at any time.
Although this device is quick and convenient for measuring personal exposure, it does not provide the sensitivity needed to accurately measure the low levels that may be encountered in a suspect environment (e.g. nuclear contaminations area). This is fully evident in that the above device provides full scale readings of 200 mR (5.16 * 10.sup.-2 C/Kg), wherein a background exposure dose rate is typically in the range of 0.014 mR/hr (3.612 * 10.sup.-6 C/Kg/hr). As such, those concerned with measuring exposure levels in such environments suspect of radiation, have long recognized a need for a dosimeter that provides the convenience of the 200 mR (5.16 * 10.sup.-2 C/Kg) Full Scale Carbon Fiber Dosimeter but with greater sensitivity to lower exposure levels.
A similar device was disclosed in U.S. Pat. No. 2,648,777, entitled "Quartz Fiber Dosimeter," issued to O.G. Landsverk on Aug. 11, 1953, and incorporated herein by reference. The Landsverk device utilizes a quartz fiber, mounted in an air chamber, that functions to indicate the real time dosage level. The chamber wall is lined with hydrogenous material having an average atomic number of 7.2 (which is the average atomic number of air), and thus provides readings in roentgens. The hydrogenous chamber wall also makes the Landsverk dosimeter sensitive to fast neutrons, and thus gives a composite reading of the gamma ray exposure and the neutron exposure. This is not desirable when separate real-time readings of the gamma dosage and the neutron dosage are required.
Although it has been known for quite some time that certain materials have different levels of sensitivity to gamma and neutron exposure, there is no existing device that has properly taken advantage of such technology to provide separate measurements of the gamma and neutron exposure. See "Status of Neutron Dosimetry," by H. H. Rossi, published in Nucleonics, September, 1952, and incorporated herein by reference. All existing devices provide a composite reading of the gamma and neutron dosage.
Therefore, it is proposed that a device that properly takes advantage of the different sensitivities inherent in different materials can provide both a combined and a separate gamma and neutron dose measurements in real-time. Moreover, a device that can provide even greater sensitivity to such gamma and neutron exposure levels is greatly desired by those skilled in the art. The present invention fulfills these needs.